There is a known technology in which it is determined that an exhaust gas processing system is defective if a signal of a sensor provided in the exhaust system does not change in the same manner as in the initial state as the quantity of reducing agent supplied to an NOx selective reduction catalyst is changed (see, for example, Patent Document 1).
However, if the quantity of supplied reducing agent becomes excessively large with the change of the quantity of the reducing agent, there is a possibility that the reducing agent will slip through the catalyst. On the other hand, if the quantity of supplied reducing agent is insufficient, there is a possibility that harmful substances in the exhaust gas cannot be removed. Furthermore, it takes time for the sensor signal to change after the changing of the quantity of supplied reducing agent. If the operation state of the internal combustion engine changes before the sensor signal changes, there is a possibility that the aforementioned determination cannot be made. In consequence, there is a possibility that the aforementioned determination will take time.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-176719    Patent Document 2: Japanese Patent Application Laid-Open No. 2008-196340    Patent Document 3: Japanese Patent Application. Laid-Open No. 2005-226504    Patent Document 4: Japanese Patent Application Laid-Open No. 2008-190529